We have previously shown that a monoclonal antibody called anti-HC2, which has specificity for hairy cell leukemia cells, also reacts with a small subset of B lymphocytes. These normal cells are activated B lymphocytes and may represent the normal cell equivalent for the leukemia hairy cells. Recent additional evidence from our own laboratory and from several other laboratories supports this conclusion. Because HC2 is expressed at a stage of B-cell differentiation, where B cells are expected to generate receptors for T-cell-derived factors, HC2 represents an excellent candidate molecule for such a receptor. We wanted to examine the possibility that HC2 represents the B-cell stimulatory factor (BSF) receptor. Using a standard BSF assay with anti-IgM as a first signal and semi-purified BSF, we have recently shown that anti-HC2 inhibits BSF-induced B cell proliferation. This year's objectives include: (1) definition of the exact function of the HC2 molecule, and (2) definition of the structure of HC2. BSF will be purified to homogeneity using HPLC. With sufficient radioactively labeled BSF a direct binding assay of the type described for IL-2 will be set up. In order to obtain full sequence and structural information on the HC2 molecule, we will initiate efforts to clone the gene for HC2. Large quantities of HC2 will be isolated by affinity chromatography using the monoclonal antibody anti-HC2 and SDS PAGE. Tryptic fragments will be isolated by HPLC and sequenced. Such sequences will allow construction of oligonucleotide probes, which may be used to screen an expression library derived from an HC2 positive B lymphoblastoid cell line. (MI)